One of the image heating apparatuses described above is an apparatus that includes an endless belt (also referred to as an endless film), a heater that comes into contact with an inner surface of the endless belt, and a roller cooperative with the heater to form a nip portion therebetween with the endless belt interposed therebetween. Continuous printing on small-size sheets using an image forming apparatus including such an image heating apparatus causes a phenomenon in which a gradual temperature rise occurs in an area of the nip portion through which the sheets do not pass in the longitudinal direction of the nip portion. This phenomenon is referred to as overheating in a no-media passage portion. Too high a temperature of the no-media passage portion may damage components in the apparatus, or may cause toner to be offset to the endless belt in an area of the large-size sheet which corresponds to the no-media passage portion.
One of the techniques to suppress the overheating in the no-media passage portion is as follows. A heating resistor (hereinafter referred to as a “heating element”) on a substrate of a heater is formed of a material having a positive temperature coefficient of resistance. Two conductors are disposed at opposite ends of the substrate in a transverse direction of the heater (a direction in which a recording sheet is conveyed) so that current flows through the heating element in the transverse direction (hereinafter referred to as the path of current in the conveyance direction) (see PTL 1). In the concept disclosed in PTL 1, as the temperature of the no-media passage portion increases, the resistance of the heating element in the no-media passage portion increases, suppressing current flowing through the heating element in the no-media passage portion and thus preventing the overheating in the no-media passage portion. The positive temperature coefficient of resistance is a characteristic in which the resistance increases as the temperature increases, and is hereinafter referred to as the PTC.
However, also in the heater described above, a certain amount of current flows through the heating element in the no-media passage portion.